1. Field of the Invention
The object of the invention is to provide an electromagnetic wave reflector with a convex surface and also concerns its production method. More specifically, this reflector constitutes the secondary reflector of a radio antenna with a "Cassegrain" type configuration, said reflector designed to function in a wavelength range extending up to 20 GHz.
2. Description of the Prior Art
In particular, these antennae are used in the field of telecommunications and may be used on land or in space. As regards spatial applications, these antennae are designed to equip telecommunications satellites.
Although the reflector of the invention is more particularly designed to constitute the secondary reflector of a "Cassegrain" type antenna, it may also be used as a reflector in a conventional single-reflective antenna or as the main reflector in a double-reflective antenna.
To help distinguish the invention from the prior art, the following figures are referenced:
FIG. 1 Cassegrain type antenna arrangement; and
FIG. 2 sectional view of antenna reflector.
An antenna with a conventional configuration is composed of a radiofrequency source and a reflector with a parabolic form whose concave face usually constitutes the active face. The source is placed at the focal point of the reflector and is designed to emit or receive electromagnetic radiation focalized by the reflector.
In certain arrays and more particularly in the space field, a secondary reflective antenna is preferably used having a "Cassegrain" type configuration so as to limit the spatial requirement of the antenna for a given focal distance (usually from 1 to 3 m). FIG. 1 diagrammatically shows a "Cassegrain" type antenna.
This antenna mainly comprises a reflector or principal mirror 2 which is a focal point paraboloid F.sup.1, a reflector or secondary mirror 4 whose surface is a focal point hyperboloid type surface F.sup.2 and a primary source 6 placed in the focal point F.sup.2.
For transmission functioning, the source 6 illuminates the secondary reflector 4 which reflects the radiation 7 onto the principal reflector 2, the latter ensuring the directivity of emission of the electromagnetic radiation.
In receiving, functioning is effected in the opposite direction: receiving of the electromagnetic waves by the principal mirror 2 which reflects these towards the secondary mirror 4 where they are again reflected towards the source 6.
The configuration represented on FIG. 1 is an "Offset" or "moved out of center" type configuration. The functioning of a "centered" type antenna is almost the same.
In spatial applications, the active face of the antenna reflectors, namely respectively the reflecting faces 4a and 2a of the principal 4 and secondary 2 mirrors, are covered with a silicon-based paint, usually white. The aim of this paint is to protect the reflectors mounted on satellites from any cyclic thermal variations caused by the alternating passages of shadow zones and solar illumination zones.
This thermal protection makes it possible to minimize any resultant thermoelastic deformations of the reflector by keeping the active faces 4a and 2a within a range of profiles, which retains the desired radioelectric performances of the antenna.
Although this paint provides a generally satisfactory thermal insulation, in certain cases it does have a number of drawbacks. These are accounted for by the fact that the incident radiation traverses the paint layer before being reflected onto the conductive surface 4a or 2a of the reflector.
In the case of a circular polarization electromagnetic wave, the paint layer provokes a phase shift between the components of the vertical and horizontal electric field. This phase shift destroys the purity of the circular polarization and the reflected radiation then exhibits an elliptic polarization corresponding to a loss of energy. This phenomenon is much more significant when the angle of incidence i (FIG. 1) made by the radiation with respect to normal at the active surface is high.
For small angles, this usually being the case in antennae with a single reflector, the effect of this phase shift cannot be taken into account. On the other hand, these disturbances are quite significant in the case of secondary reflectors "Cassegrain" type antennae and more particularly those with a "moved out of center" configuration where the angles of incidence of radiation may reach high values (about 60.degree.) on the secondary reflector.
Furthermore, as regards spatial applications, the antenna reflectors need to be as light as possible so as to facilitate the placing in orbit of a satellite equipped with these reflectors.
In order to overcome these drawbacks, an antenna reflector with a convex active face has recently been designed, as diagrammatically shown on FIG. 2. This antenna reflector 4 comprises a rigid support 10 whose active face 10a is entirely coated with the paint 12 containing a heat insulating material. This insulating layer 12 is itself covered with a metallized coating 14. In particular, this coating 14 is a polyimide film, such as (Kapton.sup.200), with a thickness of 25 micrometers, covered with a 30 to 40 nm layer of aluminum.
This coating 14 is relatively light and ensures reflection of the electromagnetic waves 7, as can be clearly seen on FIG. 2, and thus prevents electromagnetic radiation from traversing the paint layer 12 and accordingly its change of polarization.
So as to ensure a minimum weight of the reflector, the rigid support 10 is formed by a rigid honeycomb-shaped structure sandwiched between two carbon coatings 18 and 20.
The reflector of FIG. 2 makes it possible to clearly overcome these previously mentioned drawbacks.
Unfortunately, the use of an aluminized (Kapton.sup..RTM.) coating 14 has a certain number of drawbacks. In fact, this type of material is difficult to produce as it needs to be formed with a precise mechanical tension so as to absorb the volume expansions of the support 10 in a cycle of temperatures normally ranging from -160.degree. C. to +100.degree. C. where a satellite antenna is placed into orbit, whilst ensuring a proper reflection of the waves.
In addition, this coating is difficult to implement and may possibly tear or crack. Finally, this coating is slightly ductile, which limits its use. In particular, this material cannot be used for reflectors with extremely high convexity.